Everyone in my family always wears helmets when riding/driving UTVs. With kids around, I figure it's better to just get them used to always putting on a helmet rather than them seeing dad putt around the property without and thinking that's okay.
Side note, I've had 4 friends killed on motorcycles (3 street and 1 dirt) and none of them had on helmets...their families still wonder if the outcomes would've been different.
Granted, UTVs have cages that protect us most of the time in rollovers, but what if you are ejected, what if your head slaps against the ground in a roll over (seatbelts sometimes have too much slop before they lock up, giving you enough sideways motion). And I've seen more than one cage fail in roll overs...